This is one of my personal favorite philosophical debates. I was actually discussing this topic recently with a few people at school. Ultimately, we don't know if the reality we experience is actually real. Yes, we could all just be brains in a vat. We have no way of knowing. However, we can usually determine if events are true in the reality we experience by examining the evidence. We can show that events such WW2 happened in our reality through several methods, such as historical records, archaeology, etc... The Harry Potter example is actually interesting. We do know that the book is categorized as fiction but can we actually determine that the world of Harry Potter doesn't exist? Well, to an extent, yes. The problem is that in Harry Potter we would be living in the Muggle World (no magic) and Muggles can't see Hogwarts due to magical protections. So how would we know that Hogwarts doesn't actually exist? : Now before everyone gets excited about getting your own wands and flying around on brooms, I should point out that we also have no way of knowing that it IS real and have no evidence to support it's existence. We can't possibly know either way. We can ASSUME it doesn't based on what we know about our reality but we can't actually know for a fact.